This invention relates to a method for forecasting stem rot disease of rapeseed (Brassica napus or B. campestris) caused by the fungus Sclereotinia sclerotiorum (Lib.) de Bary and to media compositions used in this forecasting assay.
The major features of the S. sclerotiorum life cycle in western Canada are the following. The fungus produces resting structures (sclerotia) in and on diseased plants. These structures overwinter with plant residues at or near the soil surface and can persist several years in the soil. In late June or early July the resting structures germinate if they are exposed to suitable conditions of moisture and temperature. Adequate moisture is generally available only when a sufficiently dense plant canopy has developed. The canopy provides permanent shading of the soil which allows the surface to remain moist for at least several days at a time. Germination of the resulting structures produces mushroom like structures (apothecia) that in turn release spores (ascospores). The spores become airborne and may travel hundreds of metres in the wind.
The spores can infect susceptible plants, such as rapeseed, but only if they are provided with nutrients from dead organic material on the plant surface. Normally in rapeseed crops, airborne spores are deposited on petals in situ in the inflorescence; the petals are ephemeral and after a few days fall from the inflorescence, carrying spores with them. If the contaminated petals land on rapeseed leaves or lodge in leaf axils, and if adequate moisture is available, the fungus colonizes the fallen petals and then penetrates the plant surface and initiates an infection. The fungus spreads in the stem, causing rotting, bleaching and weakening, resulting in shrivelled seeds, premature ripening and lodging in the crop.
Control of sclerotinia stem rot with only a single fungicide application during flowering is possible because of the unique role of flowering in the disease cycle. Before flowering the crop is usually not sufficiently dense to provide the right moisture conditions at the soil surface for spores to be released. Before and after flowering, dead petals are not available on plant surfaces to promote spore germination and infection. Thus protection of the crop throughout the entire growing season is unnecessary.
Application of the fungicides used to control sclerotinia stem rot is expensive, and since all crops do not automatically become infected, there is no need for systematic spraying. Farmers need to identify crops that are at high risk of infection; however, they cannot wait until diseased plants are evident in the crop. Disease does not appear until the crop is in late bloom and by then spraying is useless. Thus, there is great interest in an effective method of forecasting disease when the crop is in the early flowering stage so that a decision whether to invest in chemical control can be made.
Historically, disease forecasting systems have been host-, pathogen- or weather-oriented or have employed some combination of these three factors (Fry, W.E., 1982, Principles of plant disease management, Academic Press, N.Y., 378 pp.; Zadoks, J.C., 1984, Plant Dis. 68: 352-355). A forecasting system employing an arbitrary scale of points has been developed in western Canada for management of stem rot of rapeseed (Thomas, P.M., 1984, Canola Growers Manual, Canola Council of Canada, Winnipeg, Manitoba, p. 1053-1055). Points are awarded for qualitative measures of crop history, crop density, potential yield, probable degree of lodging, soil moisture, presence of water in the crop canopy, weather conditions before and during bloom, and the presence of apothecia in and around the field. Fungicide application is deemed necessary if the total number of points assigned exceeds a predetermined threshold (Thomas, P.M., 1984, op.cit.). While this system provides some assessment of disease risk, it does not allow quantitative disease prediction. Provided inoculum levels could be effectively monitored, the addition of a quantitative inoculum density-disease incidence (IDDI) relationship to this forecasting system would improve its accuracy and, when combined with reliable yield loss data (Morrall et al, 1984, Can. J. Plant Pathol. 6: 265), provide an estimate of the potential dollar loss.
Recent studies in eastern Canada demonstrated significant relationships between apothecium density and disease incidence in large (105 m.sup.2) plots of white bean and soybean (Boland, G.J., 1984, Ph.D. Thesis, University of Guelph, Guelph, Ontario; Boland, G.J. and Hall, R., 1982, Can. J. Plant. Pathol. 4: 304; Boland, G.J. and Hall, R., 1983, Can. J. Plant Pathol. 5: 201). However, apothecium monitoring is not practical in non-row crops like rapeseed, especially on a large scale, because of the destructive sampling required. Furthermore, no clear IDDI relationships have yet been demonstrated for sclerotinia stem rot of rapeseed. W. Kruger's (Z. Pflanzenkrankh. Pflanzensch. 82: 101-108, 1975) statement that epidemics in winter rapeseed fields in Germany require the development of at least 3 apothecia/m.sup.2 under favourable infection conditions probably does not apply to spring rapeseed in western Canada; Morrall and Dueck (Can. J. Plant Pathol. 4: 161-168, 1982; Proc. 6th Int. Rapeseed Conf., Paris, France, 957-962, 1983) have reported severe infestations in fields with few or no apothecia. The role of extrinsically-produced ascospores in causing disease in rapeseed fields may, therefore, be of considerable importance (Hims, M.J., 1979, Plant Pathol. 28: 197-198; Morrall, R.A.A. and Dueck, J., 1982, op.cit.; Morrall, R.A.A. and Dueck, J., 1983, op.cit.; Williams, J.R. and Stelfox, D., 1979, Plant Dis. Rep. 63: 395-399; Williams, J.R. and Stelfox, D., 1980, Can. J. Plant Pathol. 2: 169-172). Accordingly, ascospore concentrations above the crop canopy and on plant surfaces might reflect the disease potential in a crop better than the density of apothecia in the field.
The improved disease forecasting system we have developed depends on a quantitative assessment of the frequency of infestation of rapeseed petals with ascospores of S. sclerotiorum when the crop is in early bloom. By collecting petals from several parts of a field and determining the frequency of infestation with S. sclerotiorum, it is possible to forecast whether the risk of disease in the crop is low, moderate or high. The actual disease outcome in the crop will depend, of course, on environmental factors that influence the process of plant infection. However, it is possible to identify fields that are at low risk, and thereby save the farmer unnecessary expensive fungicide applications. This method has broader implications as detailed below.